Today, after the attack at the Boston Marathon, Boston is a changed city. People are stunned and sad, desperate to know who would attack us and why.

The FBI is not ready to tell us much. But there are some clues about what sort of individual or group might be responsible. The type of weapon — a pressure-cooker device — is one important clue. While this kind of bomb has been used around the world, including in the Mumbai attacks of 2006, it was recently promoted in an article titled “How to Make a Bomb in the Kitchen of Your Mom” in the summer 2010 issue of al-Qaeda’s online magazine Inspire. The Times Square bomber Faisal Shahzad followed the recipe, though Shahzad’s bomb would have killed many more people than the relatively small bombs in Boston.

Who might use such a device? The first possibility would be individuals following al-Qaeda’s recipe, imagining themselves to be furthering its goals by carrying out a “do-it-yourself” attack. Al-Qaeda in the Arabian Peninsula has been publishing “open-source jihad” instructions and ideas for how to commit low-level terrorist attacks, and Westerners hoping to participate in the “jihad” are urged to carry it out at home. It’s too risky to travel to Pakistan to get trained; jihadist volunteers are too likely to get caught. Instead, volunteers are urged to carry out their own low-level, leaderless attacks.

But leaderless resistance actually has its origins in American antigovernment groups, which is the second possibility. The concept was first introduced in the 1980s in a magazine called Survivalist Alert. It was then popularized by neo-Nazis on websites like Stormfront and later picked up by groups affiliated with al-Qaeda. The purpose of leaderless resistance is to evade law-enforcement detection. If small groups, unaffiliated with the movement’s leaders, could act on their own, the virtual organization would be far less vulnerable. This style of organization has been greatly enhanced by the Internet, which allows participants to communicate anonymously. The upside, from the terrorists’ point of view, is that such groups are harder to penetrate and stop. The downside for them (and the upside for us) is that they can’t do as much damage as a terrorist army.

Another variable that makes us consider American antigovernment groups is the date of the attack. Patriots’ Day is a sacred holiday for these groups. First, it was the date, in 1985, that law-enforcement authorities invaded the compound of a white-supremacist antigovernment group called the Covenant, the Sword and the Arm of the Lord (CSA). That group, which aimed to fight the “Zionist Occupied Government,” had acquired a crude chemical device and plotted to blow up the Alfred P. Murrah Federal Building. Ten years later, Timothy McVeigh, who had been in contact with the leadership of CSA, actually carried out the attack, choosing a day close to Patriots’ Day in part because it was the anniversary of the siege of Waco.

Since the late 1960s, Patriots’ Day is celebrated in Massachusetts on the third Monday in April, which this year was on April 15, coinciding with tax day, another important day for antigovernment groups. And while the choice of weapon might make authorities focus on leaderless jihadist groups, it’s worth noting that Inspire magazine’s recipe for pressure-cooker bombs was promoted by the white-supremacist website Stormfront.

According to the Southern Poverty Law Center (SPLC), American antigovernment groups are enjoying a renaissance of sorts. In March, the SPLC sent a letter to the attorney general and the Secretary of Homeland Security warning that “we now also are seeing ominous threats from those who believe that the government is poised to take their guns.” The SPLC warns that the number of conspiracy-minded antigovernment groups grew from 149 in 2008, the year that President Obama was elected, to 1,360 in 2012. Today’s figure exceeds the highest figure from the 1990s by more than 500. The SPLC sent a similar warning to the U.S. government six months prior to the Oklahoma City bombing in 1995.

Whoever the perpetrators of this heinous crime turn out to be, it is one more reminder that there are costs to choosing to live in a free society. Totalitarian rule is a far better bulwark against terrorism (at least inside the country) than is democracy. But few Americans would want to live in the kind of country where this kind of low-tech, apparently leaderless attack could always be thwarted. Unfortunately, this dilemma will be made all the more apparent if investigators ultimately conclude that the terrorists in question were homegrown.